The present invention relates generally to a thermocatalytic reactor of the type used in a radiant heating unit and to a process for producing same. The reactor of the present invention is particularly adapted for use in a radiant heater of the flameless gas combustion type. In such heaters, a thermocatalytic reaction is sustained at or near the outer surface of a thermocatalytic reactor cylinder causing it to incandesce and produce an output of radiant energy. An example of such a flameless radiant heater including a thermocatalytic reactor is illustrated in U.S. Pat. No. 3,275,497 entitled METHOD OF MOLDING A COMBUSTION ELEMENT OF CERAMIC FIBERS ON A POROUS SUPPORT which issued on Sept. 27, 1966 to Gerhart Weiss et al., the disclosure of which is expressly incorporated herein by reference thereto.
In such radiant heaters, the thermocatalytic reactor typically constitutes a refractory cylinder enclosed at one end. A plurality of discrete, amorphous, inorganic ceramic fibers are generally arranged in the homogenous, porous wall structure of the reactor cylinder. The open end of the reactor cylinder is generally mounted on a hollow fitting which extends through a metal reflector. A combustable, air-gas mixture is passed through the center of the cylinder and the pores of the reactor cylinder wall cause the outer surface thereof, upon combustion, to incandesce. Reactors of this type will normally remain thermally stable for prolonged periods of time at relatively high operating temperatures.
The aforementioned combustion reaction in flameless and, because of the relatively low thermal conductivity of the reactor cylinder, there is no flashback of the reaction into the interior of the reactor cylinder. A relatively high percentage of the thermal output of the reactor cylinder is radiant energy.
Heretofore, combustion reactor cylinders of this type were produced by a process which included the addition of a filler or binder material added during processing to chemically bind the reactive components together. As described, for example, in U.S. Pat. No. 3,275,497, the components of the cylinders including alumina and colloidal silica were admixed in a molding bath and then dispersed in water. An aqueous solution of aluminum nitrate was then added to form a gel which was then further diluted in water. Chopped fibers formed from a melt of alumina Al.sub.2 O.sub.3 and silica SiO.sub.2 were then added to form a slurry. A preferred fiber was derived from Kaolin. A filler or binder, such as, for example, methyl methacrylate, was then added to chemically bind the alumina fibers and silica together.
The slurry was then formed into a reactor cylinder by adhesion around a screen mounted on a base which became the inner tube of the reactor cylinder. This inner tube during formation of the cylinder would typically be connected to the suction line of a pump and be immersed in the gel molding bath for a period of time sufficient to deposit an adequate amount of the gel on the screen to form the molded reactor cylinder. After removal from the bath, the cylinder was dried at a temperature between about 140.degree. F. and about 150.degree. F. for a period of time between about 10 and about 60 minutes. After drying, the reactor cylinder was then baked in a kiln at a relatively high temperature, i.e., in excess of 1100.degree. F., in order to sublime the methyl methacrylate binder. In this prior art process, the kiln heating operation was a required processing step since the binder had to be sublimed. Heating to such temperatures, however, had a deleterious affect on the alumina fibers located in the reactor cylinder. At temperatures below about 1800.degree. F., the alumina fibers were generally in either the gamma or theta phase or a combination thereof. At temperatures above 1800.degree. F., however, the alumina fibers undergo a further phase transformation into the alpha phase. As the alumina transforms from gamma to theta and then to alpha, there is a tendency for the alumina fibers to densify, thereby reducing the porosity and available surface area of the resultant reactor cylinder.
It was recognized in U.S. Pat. No. 3,275,497 that, ordinarily, the phase of the alumina fibers in the reactor cylinder had little, if any, consequence on its operational characteristics. In certain situations, however, particularly when catalytic agents were added to the bath for deposit on the surface of the fibers, the gamma phase of the alumina was preferable because of its generally higher surface to mass ratio.
As previously stated, it is preferred that the reactor cylinder be porous and provide as large a surface area as possible in order to most efficiently serve as a combustible source. Accordingly, whenever possible, it is preferable that the alumina in the reactor cylinder be primarily in the gamma or theta phases rather than in the denser alpha phase. This was heretofore impossible due to the fact that the binder had to be sublimed at temperatures in excess of 1100.degree. F. which resulted in a reactor cylinder which was denser and less porous than optimally preferred.
The process of the present invention attempts to provide a more optimal reactor cylinder formed without a binder. This permits elimination of the step of heating the cylinder in a kiln at a temperature in excess of 1100.degree. F. in order to sublime the binder, which thereby produced a denser and less porous reactor than optimally preferred. This has been accomplished by the addition of powdered talc. The resultant binderless reactor cylinder formed at ambient temperature, therefore, includes alumina fibers in the less dense gamma phase. Accordingly, the reactor cylinder of the present invention is more porous and has a greater surface to weight ratio than reactor cylinders heretofore used.
It is therefore, a primary object of the present invention to provide a binderless thermocatalytic reactor cylinder for use in a radiant heater of the flameless gas combustion type.
It is another object of the present invention to provide such a reactor cylinder which includes powdered talc as a component thereof.
It is still another object of the present invention to provide such a reactor cylinder which is more porous and has a greater surface area than the binder-containing, reactor cylinders heretofore used.
It is yet still another object of the present invention to provide such a reactor cylinder which includes alumina fibers primarily in the gamma phase.
It is still yet another object of the present invention to provide a process for producing such a reactor cylinder.